1. Field of the Invention
The present invention relates generally to an image input device employed in an image reading apparatus such as an image scanner apparatus or the like.
2. Description of Related Art
For having better understanding of the concept underlying the invention, description will first be made of a conventional image input device known heretofore. FIG. 9 of the accompanying drawings is a block diagram showing generally and schematically an arrangement of a conventional image input device. As can be seen in this figure, the image input device is composed of a scanning unit 61 and a scanner driving unit 66.
The scanning unit 61 is comprised of an original reading module 62, a resolution converting module 63, an image processing module 64 and an image type transforming module 65. On the other hand, the scanner driving unit 66 includes a read range designating module 67, a resolution designating module 68, an image processing designating module 69 and an image type designating module 70.
The original reading module 62 constituting a part of the scanning unit 61 includes a light source (not shown) for reading the image from an original such as a document or the like. More specifically, the original is illuminated with light emitted from the light source, and the light rays reflected from or transmitted through the original are converted into an analogue electric signal through a photoelectric conversion. The analogue electric signal thus obtained then undergoes an analogue-to-digital conversion (A/D conversion for short) by an A/D converter (not shown). While performing the operation mentioned above on a line-by-line basis in a primary scanning direction within an image read range set previously, the original reading module 62 is moved sequentially in a secondary scanning direction with the image data being outputted.
The read range designating module 67 constituting a part of the scanner driving unit 66 is generally so designed as to determine an image reading range with the aid of the user. On the other hand, the resolution designating module 68 serves for determining the reading resolution of the scanning unit 61 (i.e., resolution with which the scanning unit 61 is to read the image from the original). Further, the image processing designating module 69 determines the processing to be performed on the image data upon reading thereof. In addition, the image type designating module 70 indicates or designates an image type in the image reading operation. Individual items indicated or designated by the modules 67, 68, 69 and 70 are set at the scanning unit 61. Thus, the scanning unit 61 can execute the processing for reading the original in accordance with the designated items as set. In this way, an output image demanded by the user can be generated.
At this juncture, description will be directed to a resolution converting process in the conventional image input device. It is assumed, by way of example, that image data having an optical resolution of 600 dpi outputted from the scanning unit 61 is to be converted into image data of another resolution which lies within a range of 60 to 2400 dpi.
Conversion of the resolution of the image data in the primary scanning direction can be achieved generally by resorting to a digital differential analysis algorithm (DDA algorithm for short). FIG. 10 of the accompanying drawings is a view for illustrating a resolution converting processing in the primary scanning direction performed in the scanning unit. More specifically, there are shown in FIG. 10 straight lines having respective slopes which correspond to different resolutions relative to a vector for which the ratio between the number of the input pixels and that of the output pixels is 1:1 (e.g. the case where the resolution is 600 dpi in both the input image and the output image). In more detail, in FIG. 10, there are illustrated, by way of example, resolutions of 240 dpi (40%) and 900 dpi (150%) in the output images, respectively.
In conjunction with conversion of the resolution in the primary scanning direction from 600 dpi to 240 dpi, there can be derived the below mentioned relation (1) in calculating the input pixels x for the output pixel y after the conversion in accordance with "x=2.5y" which represents a reverse function of "y=0.4x" shown in FIG. 10. Namely, EQU S1=25.4 mm/(60.multidot.T) mm/sec.gtoreq.feeding speed.gtoreq.S2=25.4 mm/(2400.multidot.T) mm/sec (1)
The above expression (1) indicates that 2.5 pixels are generated from the input pixel for a first output pixel y=1 for allocation thereto while for a second output pixel y=2, five pixels are allocated.
In this way, the pixels are generated from the input pixel at the ratio given by the above expression to be outputted as the output pixels.
On the other hand, for conversion of the input image having the resolution of 600 dpi into an image having the resolution of 900 dpi, there can be derived the undermentioned relation for calculation of the input pixels x for each output pixel y in accordance with a function "x=0.67y" which is reverse to "y=1.5x" shown in FIG. 10. EQU (y, x)=(1, 2.5), (2, 5.0), (3, 7.5), (2)
At this juncture, it should be mentioned that for determining the image data for the fragmentary pixel, a weighted mean of two adjacent pixels is used. For instance, 1.33 pixel can be determined as follows: EQU (y, x)=(1, 0.67), (2, 1.33), (3, 2), (3)
As is apparent from the above description, when the input image is converted into an image of lower resolution, the input image data is subjected to a thinning processing whereas in conversion of the input image into an image of higher resolution, a thickening processing is effectuated.
The resolution converting processing in the secondary scanning direction can be effectuated by translating the distance for which the original reading module 62 is displaced in the secondary scanning direction during a charge storing period (T sec.) taken for the photoelectric conversion by the original reading module 62. More specifically, the conversion of resolution is realized by changing the feeding speed of the original reading module 62 within the range of S1 to S2 corresponding to the resolution range of 60 to 2400 dpi in accordance with the following expression (4): EQU (first pixel).times.0.67+(second pixel).times.0.33 (4)
As is apparent from the above description, with the conventional image input device, the resolution converting processing of the image data is carried out internally of the scanning unit 61. As a consequence, the structure of the scanning unit 61 tends to be very complicated, involving difficulty in fabricating the scanning unit 61 at low cost.
Further, in the conventional image input device, the resolution interpolating (converting) processing is performed by thinning or thickening the data hardwarewise in the primary scanning direction. On the other hand, conversion of the resolution in the secondary scanning direction is carried out by controlling variably the distance for which the image read module is moved on a line-by-line basis. Consequently, the range of resolutions susceptible to the conversion is limited by limitation imposed on a motor feeding control, giving rise to a problem.
Of course, it is certainly possible to realize conversion of the resolution in the secondary scanning direction by resorting to other method than the motor feeding speed control. In that case, however, it will become necesily to increase the capacity of the line buffer memory, which results in a complicated circuit configuration and hence complicated structure of the device.